1. Technical Field
The present invention relates to a method for joining metal materials at least one of which has an oxide film on the surface, in more detail to a low-cost method for joining the metal materials that can be performed in the air at low temperature and can minimize the influence of heat on the base materials and surroundings.
2. Related Art
Various metal materials are used in industrial products, and such metal materials are joined by various types of joining methods including joining between heterogeneous metal materials as well as joining between homogeneous materials. For example, copper- or aluminum-based metals, which have low electrical resistance, are used for parts of various batteries and electric/electronic devices. In these parts, such metal materials would be joined to each other. Further, for the purpose of reducing contact resistance, gold- or silver-plated parts would also be joined.
Metal materials as described above used in such parts can be joined by brazing with a solder. However, the resulting parts cannot be used at a temperature higher than the melting point of the solder (brazing material) (e.g. around 200° C.).
Further, even though it is possible to improve the use temperature of parts by using a high-temperature solder, solders containing a noble metal increases the cost. Further, although it is depending on the composition of a solder, intermetallic compounds or a Kirkendall void may be formed in a high temperature condition, which impairs the strength and the durability. For these reasons, it is desirable that metal materials as described above are directly joined to each other without using a brazing material.
On the other hand, the above-described metals except for gold typically have an oxide film on the surface. Such oxide films block direct joining, and thereby make metallurgical joining difficult.
For example, Patent Document 1 describes a method of joining aluminum to aluminum or aluminum to alumina, including: interposing an insert containing an element that causes eutectic reaction with the base material between faces to be joined, making them in contact with each other in an oxygen atmosphere, and then heating the faces to be joined to a temperature in the range that effects the eutectic reaction, so that a melt phase on the contact face due to the eutectic reaction as well as an oxide phase due to a reaction between the components of the base material and oxygen present at the gap of the contact faces are produced (see claim 1). As a result, it is described that the oxide films on the surfaces of the base material are broken and mixed in the melt phase along with the oxide produced by a reaction between the components of the melt phase and oxygen (see the middle part of the left column on page 3).